Let's Go Gardening
General gardening site, with versions geared towards US and UK gardeners. Tips and articles, informational resources, links to events, societies, and public gardens, and a forum.
http://www.letsgogardening.co.uk/

LoveToKnow Garden
Short informational articles on gardening topics, vegetables, and ornamental plants, contributed and edited by users.
http://garden.lovetoknow.com

Hort.net
Offers a collection of garden reference materials, including a plant gallery and profiles, archives of mailing lists, gardening news, and links.
http://www.hort.net/

MyGardenGuide
Offers plant database with photos and summary information, Q&A on various garden topics, and a garden blog.
http://www.mygardenguide.com/

Garden Terms
Gardening dictionary with brief definitions of a large number of terms, optionally arranged by category.
http://www.gardenterms.com/

Mimi.hu: Gardening
Provides a directory of biological and horticultural terms, with links to online encyclopedia entries for each term.
http://en.mimi.hu/gardening/index_gardening.html

Yardener.com
Lawn and garden resources for homeowners whose garden is not their hobby. Includes information and care data for many plants, shrubs, and trees, and dealing with pests and problems.
http://www.yardener.com/

Garden Ideas
Articles and advice on a variety of gardening topics.
http://www.gardenideas.com/

Pioneer Thinking: Gardening Center
Information and tips on outdoor and indoor gardening. Many articles grouped by topic.
http://www.pioneerthinking.com/gardening.html